This invention relates to data processing systems and more particularly, to data processing display systems including one or more display devices each capable of displaying a desired image.
Prior art processing display systems are characterized by special purpose memories which in the aggregate define a character generator. Typical display devices are of the raster-scanned type, such as CRT-TV tubes. A character to be formed at a location on the display screen is defined by a matrix of selectively energizable points arranged in a plurality of vertically oriented scan lines. As is conventional, a scanning beam scans from left to right, one scan line at a time, until an entire "page" is displayed, and then returns to the top scan line in order to "refresh" the image on the screen. Typically, the image is refreshed 30 times per second.
It is a characteristic of prior art systems that characters are definable only in fixed "blocks" on the display screen, each block containing the requisite matrix of energizable points. As is conventional, a display list memory contains the character data to be displayed in the form of a plurality of ASCII coded words each defining a particular character element. A predetermined number of words in the display list memory define each text line to be displayed. Each ASCII coded word from the display list memory is forwarded to a separate font memory along with a signal indicating a scan line number. For example, there may be eight scan lines in the matrix for each character which then could be 8 .times. 8. Thus, the scan line number signal could be a 3-bit code. The font memory contains a plurality of words each representing the desired bit configuration for each scan line of a particular character. It is the output of the font memory that is supplied to the display controller for transmittal in bit-serial format to the display device. The words of the font memory are traditionally supplied directly one at a time to the display controller. Other prior data processing display systems, such as that disclosed in U.S. Pat. No. 3,895,357, store a predetermined number of the font memory words, such predetermined number defining a small band of the picture to be displayed, into a partial raster assembly store. Again, however, special purpose memories are employed.
The main problem with these prior art "character generator" type display systems is that they are generally limited to forming characters only, and only on specifically predetermined portions or "blocks" of the display screen. They do not have a "graphics" capability, i.e. the ability to draw relatively high resolution lines and curves unrelated to the conventional alphanumeric characters. U.S. Pat. No. 3,716,842 discloses a data processing display systems that does have a graphics capability. However, the system is relatively complex from a hardware standpoint in requiring, again, special purpose memories.
It would be desirable, therefore, to provide a data processing display system having the attributes of simplicity and generality, i.e. simplicity in a hardware sense such as by avoiding special purpose memories, and generality in a display capability sense such as being virtually unlimited as to the nature and location of data displayed on the display thereby providing a graphics capability.